An essential problem in radio systems is the rapid variation of radio channel properties as a function of time. This concerns for example mobile systems, in which at least one of the parties involved in a connection is often mobile. Thus, the impulse response of the radio channel varies within a wide phase and amplitude range, up to thousands times a second. This phenomenon is random by nature, and can therefore be mathematically described using statistical means. In addition, changing distances causes varying delays to the impulse responses. Such phenomena complicate the design of radio connections and the apparatuses to be used.
There are many reasons for the variation of a radio channel. When transmitting a radio frequency signal from a transmitter to a receiver on a radio channel, the signal propagates along one or more paths, in each one of which the phase and amplitude of the signal vary, thus causing fades of different lengths and strengths to the signal. In addition, noise and interference from other transmitters also disturb the radio connection as well as the changing distances.
A radio channel can be tested in either actual conditions or using a simulator that simulate actual conditions. Tests conducted in actual conditions are difficult, since tests being carried out outdoors, for instance, are affected for example by the constantly changing weather and season. Even measurements carried out in the same place provide different results at different instants of time. In addition, a test conducted in a particular environment, such as a city, does not fully apply to a test conducted in another city. In general, the worst possible situation cannot either be tested in actual conditions.
FIGS. 1A and 1B show an impulse response pattern of a radio channel at two different instants of time. In the Figures, x-axis describes the delay of the signal components and y-axis depicts the amplitude, or energy, of the signals. The Figures also illustrate that the number of impulse response taps, the delays and energies vary as a function of time as a result of the changes occurring on the radio channel, and even though minor differences are found between FIGS. 1A and 1B, a fairly static channel is concerned. The coordinates in FIG. 1C, in turn, show the mean energy of the impulse responses formed at different instants of time. FIGS. 1A and 1B show that several impulse responses are used when calculating the mean energy. FIG. 1C illustrates the allocation of FIR blocks 100A to 100C in a FIR filter (Finite Impulse Response) to be used when forming an impulse response in relation to a delay axis. In prior art allocation is carried out by analysing the energy mean in advance, and by covering all significant energy components with FIR blocks in the actual simulation.
FIGS. 2A to 2B illustrate the impulse responses at two different instants of time. FIGS. 2A and 2B show what is known as a sliding tap caused by a large reflecting change in the terrain affecting the radio connection, for instance, and by a terminal, such as a mobile phone, diverging in relation to a particular location in the terrain. FIG. 2C illustrates the energy mean as a function of time. The Figure shows that the sliding tap is illustrated as a low energy level within a wide delay range in an average energy pattern. In order to cover the entire energy distribution on the delay axis, FIR blocks 100A to 100F are correspondingly allocated in accordance with the prior art over the entire delay range.
A significant drawback associated with prior art allocation is that the apparatus resources must be allocated in vain when particular channel models are concerned. An example of such a channel model is the sliding tap channel model.